The present invention relates to a method of fabricating an electrical contact in a semiconductor device.
In the fabrication of integrated circuits, silicon is commonly used in its monocrystalline form as a substrate and in its polycrystalline form for gate electrodes and interconnects. Aluminium and aluminium alloys are commonly used as a conductor and especially for forming contacts for interconnects. One problem of using aluminium and aluminium alloys is that silicon is soluble in or diffuses into aluminium. Thus, when a contact is formed by opening a contact window to, for example, a source or drain region formed in a silicon substrate and aluminium is used to fill the contact opening, some of the silicon disolves, resulting in what is generally called a "spike" which extends downwardly from the contact. If the spike goes all the way through the source/drain region or other underlying region, then the transistor will be ruined by a "spiked junction". The spike also results in a localised region of increased current density at the contact which can cause device failure.
To prevent this, titanium nitride can be used as a metallurgic barrier against the reaction between the silicon substrate and the aluminium contact material. Thus, when the aluminium is separated from the silicon by a titanium nitride layer, spiking is reduced or stopped entirely. It is possible to form titanium nitride by reacting pure titanium with nitrogen but it is difficult nevertheless to obtain a good barrier quality. First, one desires a barrier layer which is not too thin and second the barrier layer formed of titanium nitride should have trace amounts of impurity. It is believed that impurities such as oxygen incorporated in the barrier improve the barrier quality by inhibiting diffusion of silicon atoms through the barrier along the titanium nitride grain boundaries. Also, the barrier layer should not be in a state of tensile stress, as a stressed layer is believed to be less effective in preventing diffusion of silicon and so lower tensile or compressive stress is preferred.
U.S. Pat. No. 4,784,973 discloses that when titanium nitride is used as a diffusion barrier material, the performance of the titanium nitride as a metallurgic barrier can be enhanced by growing a thin oxide layer in the contact region prior to forming titanium nitride. The presence of a thin oxide between the silicon and the titanium during thermal nitridation reduces the rate of titanium silicide formation without affecting the rate of nitride formation and the net effect is to produce more titanium nitride and less TiSi.sub.x for the same thickness of sputtered titanium. The consumption of the oxide during nitridation leads to some silicide formation and consequently low contact resistance and the consumed oxide will effectively "stuff" the titanium nitride diffusion barrier improving the barrier properties. Conventional processes use a furnace tube operation to grow this thin diffusion barrier oxide and such an operation is disclosed in U.S. Pat. No. 4,784,973 although that specification does also disclose that the thin diffusion barrier oxide can be grown in a rapid thermal processor.
The process disclosed in U.S. Pat. No. 4,784,973 suffers from the disadvantage that it is difficult to control the formation of the thin diffusion oxide layer resulting in variability not only in the thickness of the diffusion oxide layer but also in the cleanliness thereof, leading to a reduction in device reliability. Furthermore, device reliability can also be,, reduced as a result of poor step cover and this can lead to the spiking phenomenon referred to above. The process also results in contacts having relatively high contact resistance.
Various Al alloys are used as interconnect materials for VLSI devices due to the low resistivity of the Al alloys and compatibility with conventional photolithographic techniques. For devices using more than one interconnect layer (i.e. in multilevel metallization) it is advantageous to use a "capped" Al alloy in which a mechanically strong cap material is deposited over the Al alloy which reduces the growth of hillocks (being vertical extrusions extending upwardly from the metal Al surface) during heat cycles after metal deposition. Hillocks are formed to relieve a large amount of compressive stress in the aluminum layer caused by the difference in thermal expansivity between the metal layer and the silicon substrate. The presence of a hillock can result in an unwanted connection or bridge between two layers of interconnect. A thin (around 500 Angstroms thick) layer of titanium has been shown to be effective in reducing the number and size of hillocks to an acceptable level for multilevel metallization applications. However, it has also been noted that the titanium cap reacts with the aluminium and the silicon to form a ternary compound (Al.sub.x Si.sub.y Ti.sub.z) which has a large affinity for silicon and the formation of this compound will leave a silicon doped aluminium alloy deficient of silicon.